An electronic key system wirelessly transmits a polling signal at predetermined time intervals into a predetermined transmission area outside a vehicle interior and detects an electronic key by receiving a response signal transmitted from the electronic key that is in the transmission area in response to the polling signal, and causes a predetermined function of an on-vehicle apparatus to be executed when the electronic key is detected. The electronic key transmits the polling signal into an enlarged transmission area which is enlarged from the predetermined transmission area when a state where the electronic key is detected and a state where the electronic key is not detected are repeated a predetermined number of times or more within a predetermined time, and does not cause the predetermined function to be executed when the electronic key is detected from the enlarged transmission area.